Drop `start_date` Column From Vulnerabilities

What does this MR do and why?

start_date is an ignored column (source)

This MR drops the column for step 2 of the 3-MR drop column process

Check for indexes or constraints

Using the command \d+ vulnerabilities on postgres.ai indicates there are NOT indexes and constraints

index	fk constraint

migration output

bundle exec rails db:migrate output

~/gitlab-development-kit/gitlab$ bundle exec rails db:migrate
main: == [advisory_lock_connection] object_id: 182740, pg_backend_pid: 246478
main: == 20240101133628 RemoveStartDateColumnFromVulnerabilities: migrating =========
main: -- remove_column(:vulnerabilities, :start_date)
main:    -> 0.0010s
main: == 20240101133628 RemoveStartDateColumnFromVulnerabilities: migrated (0.0036s) 

main: == [advisory_lock_connection] object_id: 182740, pg_backend_pid: 246478
ci: == [advisory_lock_connection] object_id: 182980, pg_backend_pid: 246480
ci: == 20240101133628 RemoveStartDateColumnFromVulnerabilities: migrating =========
ci: -- remove_column(:vulnerabilities, :start_date)
ci:    -> 0.0008s
ci: == 20240101133628 RemoveStartDateColumnFromVulnerabilities: migrated (0.0069s) 

ci: == [advisory_lock_connection] object_id: 182980, pg_backend_pid: 246480

bundle exec rails db:rollback:main output

~/gitlab-development-kit/gitlab$ bundle exec rails db:rollback:main
main: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 247014
main: == 20240101133628 RemoveStartDateColumnFromVulnerabilities: reverting =========
main: -- add_column(:vulnerabilities, :start_date, :date)
main:    -> 0.0010s
main: == 20240101133628 RemoveStartDateColumnFromVulnerabilities: reverted (0.0040s) 

main: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 247014

bundle exec rails db:rollback:ci output

~/gitlab-development-kit/gitlab$ bundle exec rails db:rollback:ci
ci: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 247476
ci: == 20240101133628 RemoveStartDateColumnFromVulnerabilities: reverting =========
ci: -- add_column(:vulnerabilities, :start_date, :date)
ci:    -> 0.0011s
ci: == 20240101133628 RemoveStartDateColumnFromVulnerabilities: reverted (0.0075s) 

ci: == [advisory_lock_connection] object_id: 182380, pg_backend_pid: 247476

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #268154 (closed)